The present invention relates to induction heating, and in particular, to a method and apparatus for sintering powdered metallurgical workpieces using induction heating.
Powdered metallurgical parts are widely used in preference to other manufacture to reduce subsequent sizing and forming operations and to obtain properties not obtainable with conventional materials. Powdered metallurgy finds particular benefits with respect to ferrous parts. Therein, powdered iron, alloying agents, lubricants and additives are homogenously mixed to a controlled particle size and distribution. Thereafter, the powdered composition is compacted or pressed to the shape required for subsequent forming or sizing operations. The green compact or unsintered article is then subjected to controlled heating conditions in a sintering operation for removing of volatile constituents, including die lubricants or waxes, and bind together the particles and impart the desired properties thereto. Such sintering requires accurate control of time, temperature and environment. The sintered parts are then directed to further machining, pressing and other forming and sizing operations.
Conventionally, the sintering operation is conducted in radiant batch-type furnaces or in continuous electrical or fired furnaces. In both types, a suitable non-oxidizing or reducing gas is supplied to maintain the required sintering operation. The radiant and conductive modes are not particularly thermally efficient resulting in heating times which are typically quite long. The batch-type furnace is not amenable to machined paced operation particularly where hot forming operations are required. Therein, the parts must be reheated and fed to the forming operations in accordance with production demand. The continuous furnace, in addition to being quite large and space consuming, has a limited ability to pace sintered production with the hot forming production rate. This oftentimes requires supplemental heating and handling operations. Further, the environmental reducing gas is applied to both the sintering and preheating zones. This can result in contamination of the reducing atmosphere and impairment of product quality. It also requires a gas flow rate sufficient to exhaust the volatiles and to maintain the reducing environment at the requisite purity. Thus, the relatively expensive reducing gas, required only for the actual sintering, is consumed as an effluent effectively preventing recycling and significantly contributing to the processing costs.